Various types of radiation detectors, such as silicon PIN diode detectors or silicon drift detectors, are used for measuring the energy of incoming x-ray photons. A PIN diode can be used for collecting charge carriers that are proportional in number to the energy of the x-ray photon. An example of a PIN diode is illustrated in FIG. 1, shown generally at 10. The PIN diode is comprised of a substrate 12, often referred to as an intrinsic region, an anode 11 on one surface of the substrate, and a cathode 13 on the opposite surface of the substrate. A PIN diode may be used for collection of electron-hole pairs released in response to an x-ray photon. For example, an x-ray photon may interact with an atom in the substrate, resulting in the generation of an electron-hole pair. This initial electron-hole pair may then quickly give rise to a cloud of electron-hole pairs. The electrons can travel to the anode 11 and the holes to the cathode 13. A disadvantage of the PIN diode is the large capacitance due to the large anode size. Such capacitance can result in undesirable electronic noise, resulting in poor resolution.
An example of a silicon drift detector is illustrated in FIG. 2 and shown generally at 20. A silicon drift detector, hereinafter SDD, has a small anode 25 (small relative to a PIN diode anode) at one surface of the substrate 12 and an entrance window layer 26 at the opposite surface of the substrate. Use of a smaller anode results in lower capacitance and thus less undesirable electronic noise, resulting in improved resolution. The anode can be surrounded by multiple doped rings 21. The doped rings are biased in such a way that they result in an electric field which causes electrons to flow towards the anode. The doped rings 21 can have the same doping or conduction type as the entrance window layer 26. The anode 25 can have the same doping or conduction type as the substrate 12, but usually the anode 25 is more highly doped than the substrate 12.
The doped rings 21 can be electrically coupled within the SDD. For example, a MOSFET structure 27 on an SDD is shown in FIG. 3. Conductive contacts 37 can be attached to the doped rings 21. The conductive contacts can be metallic. An insulative layer 38 separates the conductive contacts 37 from the substrate 12. The overlap 39b and c of the conductive contacts 37 over an adjacent doped ring 21 can induce a region of charge carriers 36 in the substrate 12 beneath the insulative layer 38. The charge carriers 36 in this region are of the same type as the majority carriers in the doped rings 21 and thus form a conductive path between the doped rings. For example, as shown in FIG. 3, doped ring 21c is attached to a conductive contact 37c. The conductive contact 37c overlaps an adjacent doped ring 21b at overlap 39c. Due to the conductive contact 37 attachment to one ring and overlap of an adjacent ring, when a voltage is applied across a series of doped rings, the region of charge carriers 36 can be induced in the substrate resulting in a conductive current path between the rings.
The prior art embodiment just described, in which a conductive contact 37, that is attached to one doped ring 21, overlaps an adjacent doped ring, is one method of electrical coupling. Another method of electrical coupling is shown in FIG. 4. A doped region 41, having the same conduction type as the doped rings 21, is created in the surface of the substrate and connects the doped rings 21. Conductive contacts 47 can be attached to the doped rings 21. The doped region 41 provides electrical coupling between the doped rings 21.
As shown in FIG. 2, one voltage bias V1 can be applied to the innermost doped ring that is closest to the anode 25 and another voltage V2 can be applied to the outermost ring. Because the rings are electrically coupled, the voltages at the innermost and outermost rings can create a voltage gradient across all of the rings. Another voltage V3 can be applied to the entrance window layer 26.
The voltage applied to the entrance window layer V3 can be similar in magnitude to the voltage V2 on the outermost ring. The voltage on the innermost ring V1 can have a lower absolute value than the voltage at the outermost ring V2 or at the entrance window V3. Due to the voltage gradient across the rings and the voltage applied to the entrance window 26, the charge carrier can be drawn towards the anode 25. If V2 and V3 are more negative than V1 and V1 is more negative than the anode, then an electron cloud resulting from an impinging x-ray photon can be directed to the anode. Although the prior art SDDs can have reduced electronic noise compared with the prior art PIN diode, such SDDs with electrically coupled rings can be costly to manufacture.